The first time you open an attachment, a dialog will appear where you can specify the external app, and check a box to have the setting stored in Files & Attachments.

There is no dialog, the attachment automatic open in new tab.

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, delete handlers.json to reset the Files & Attachments settings. When TB is restarted, do you see an entry in F&A to preview pdf, as in the attached picture? If so, click the drop-down to select an external app.

Dialog appeared, thanks. It is interesting that the list is still empty, but at least the pdf can be opened with an external program.